1. Field of the Invention
The invention relates generally to electron microscopy and, more particularly, to an electron microscopy image sample having a silicon nitride (Si3N4) passivation layer.
2. Description of the Related Art
Electron microscopy has been used to image semiconductor device cross-sections so that process characteristics, such as deposition rates, thicknesses, interfaces, via or step coverage, and other particular features, can be studied.
Either transmission electron microscopy (TEM) or scanning electron microscopy (SEM) has been used. TEM provides better resolution than SEM and its use has become more common as semiconductor device dimensions have become smaller. TEM is especially useful to study thin film process characteristics. However, TEM sample preparation is more complicated than SEM sample preparation.
In the conventional electron microscopy sample preparation, a spin-on-glass (SOG) layer is disposed on top of the semiconductor device being studied. Generally, the semiconductor device being studied includes at least a metal layer as the top layer and a silicon base. The SOG layer protects the metal layer from mechanical damage and functions to hold all of the layers together during the subsequent cutting process. The cutting process is necessary so that the cross-section of the sample can be exposed for imaging by the electron microscope.
Conventionally, the SOG layer is disposed on top of the study sample in the following manner. First, SOG is liquefied using a solvent. Second, the liquid SOG is dripped on top of the sample. Third, the sample is spun, forcing the liquid SOG to disperse evenly on top of the sample. Fourth, the liquid SOG is cured at temperatures above 400xc2x0 C. to remove the solvent from the SOG. At this curing stage, the SOG is subject to high enough temperatures ( greater than 400xc2x0 C.) where it reacts with part or all of the underlying metal layer to produce low quality TEM/SEM samples.
An object of this invention is to provide a method for preparing an improved sample for examination using either TEM or SEM.
The sample preparation method according to the present invention includes the steps of depositing a metal layer on top of a substrate, depositing a passivation layer comprising silicon and nitrogen on top of the metal layer, and cutting the substrate and the metal and passivation layers to expose their cross-sections for examination by electron microscopy.
The above method produces a TEM/SEM sample having sharp, well-defined boundaries. Consequently, it facilitates the study of process characteristics, including deposition rates, thicknesses, metal-substrate interface, and via or trench sidewall and bottom coverage. The method according to the present invention is especially effective when studying process characteristics of thin and very thin metal films.
Another object of the invention is to provide an electron microscopy image sample having sharp, well-defined boundaries. The sample according to the invention includes a substrate, a metal layer disposed on top of the substrate, and a passivation layer comprising silicon and nitrogen disposed on top of the metal layer, and has its cross-section cut for imaging by either transmission or scanning electron microscopy.
In the invention, the substrate underlying the metal layer may be a semiconductor base, a superconductor base, or any other base on top of which thin metal films are deposited. The semiconductor base may be, for example, a silicon base or a silicon base having one or more layers, e.g., SiO2, Si3N4, or polysilicon layer, disposed on top of the silicon base. The superconductor base may be, for example, tantalum aluminide (TaAl2O3).
The metal layer comprises one of the following layers: titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), copper (Cu), tungsten (W), tungsten nitride (WNx), and other layers conventionally used as metal layers in semiconductor process technology including other barrier-metal layers. A metal layer is considered to be xe2x80x9cthinxe2x80x9d when it is less than about 1000 angstroms. A metal layer is considered to be xe2x80x9cvery thinxe2x80x9d when it is less than about 100 angstroms.
The passivation layer comprises silicon nitride. A TEM/SEM sample employing silicon nitride layer as the passivation layer has sharp, well-defined boundaries because the silicon nitride layer is less prone to react with the underlying metal layer than a SOG layer, especially when the silicon nitride layer is deposited at temperatures less than 400xc2x0 C.
Preferably, the silicon nitride layer is deposited at low temperatures, between 300 and 350xc2x0 C., to minimize any reactions between the silicon nitride layer and the underlying metal layer. To carry out the low temperature deposition, plasma-enhanced chemical vapor deposition (PECVD) is employed.
Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows.